It is a common practice to manufacture mineral fibers, such as glass fibers, by supplying molten mineral material from a furnace or melter to fiberizers. The fiberizers typically comprise spinners for centrifuging the molten mineral material into mineral fibers. The fiber production is sometimes facilitated by the use of annular burners or internal burners which keep the spinner at proper operating temperatures. Also, annular blowers can be positioned downstream from the spinner to assist in attenuation of the fibers to the desired diameter.
One of the problems which has long plagued the manufacturers of mineral fibers is variation in the amount of glass flowing into the spinner, or "throughput". Variations in throughput create different forces on the glass fiber forming process, resulting in poor product quality because of changes in diameter of the fibers being produced. The problem of measuring throughput has been solved and, now that throughput can be measured, the throughput can be controlled at a constant by throttling the flow of molten material entering the fiberizer. Typically, the flow of molten material is throttled by changing the temperature of the bushing which emits the molten material from the forehearth to the spinner.
There is another problem, however, which still has a great effect on the manufacturing process. Even though the throughput can be maintained at a constant, variations in the viscosity of the molten mineral material can greatly affect the mineral fiber forming process. These variations in viscosity can occur because of temperature changes in the molten mineral material being supplied to the spinner, or can result from changes in the chemical composition of the molten mineral material itself. Typically, the viscosity of the molten material in a production furnace is measured by a slow, off-line process, and this measurement is usually made no more than once daily. There is a need for a viscosity measuring process which can be used on-line and done automatically, and which will not disturb the mineral fiber manufacturing process.